In various multistage turbomachines used for energy conversion, such as gas turbines, a fluid is used to produce rotational motion. Referring to FIG. 1, an axial flow gas turbine 10 includes a compressor section 12, a combustion section 14 and a turbine section 16 arranged along a horizontal center axis 18. The compressor section 12 provides a compressed air flow to the combustion section 14 where the air is mixed with a fuel, such as natural gas, and ignited to create a hot working gas. The turbine section 16 includes a plurality of turbine blades 20 arranged in a plurality of rows. The hot gas expands through the turbine section 16 where it is directed across the rows of blades 20 by associated stationary vanes 22. The blades 20 are each configured as a blade assembly that is attached to a shaft that is rotatable about the center axis 18. As the hot gas passes through the turbine section 16, the gas causes the blades 20 and thus the shaft to rotate, thereby providing mechanical work. Each row of blades 20 and associated vanes 22 form a stage. In particular, the turbine section 16 may include four rows of blades 20 and associated vanes 22 to form four stages. The gas turbine 10 further includes an exhaust cylinder section 24 located adjacent the turbine section 16 and an outer diffuser section 26 located adjacent the exhaust cylinder section 24.
The blades or airfoils 20 and vanes 22 are directly exposed to the hot gases as the gases pass through the axial gas turbine 10. Blades 20 and vanes 22 in the turbine section 16 are typically provided with internal cooling circuits that guide a coolant, such as compressor bleed air, through them to locally impinge on their internal metal surfaces, thus providing sufficient cooling to ensure part life. In certain scenarios, these cooling circuits may ultimately exit into the gas path through various film cooling holes that are formed on the surface of airfoil. The air is then discharged to the outside of the airfoil to form a film of air that cools and protects the airfoil from hot gases. Film cooling effectiveness is related to the concentration of film cooling fluid at the surface being cooled, the shape of the holed and other factors. In general, the greater the cooling effectiveness, the more efficiently the surface can be cooled. An increase in cooling effectiveness causes greater amounts of cooling air to be used in order to maintain a desired cooling capacity, which may cause a decrease in engine efficiency.
In addition, sections of the turbine 10 that form a hot gas path may include a ceramic-based coating that serves to minimize exposure of the base metal of a component, such as an airfoil base metal, to high temperatures that may lead to oxidation of the base metal. Such a coating may be a known thermal barrier coating (TBC) that is applied onto a bond coating (BC) formed on the base metal.
During operation of the turbine 10, the cooling holes may become clogged or blocked. This compromises ability to cool an airfoil surface, which may lead to undesirable base metal overheating. Moreover, spallation and/or delamination of the TBC layer or both the TBC and BC layers may occur during operation of the turbine. This also exposes the base metal to high temperatures, which may lead to oxidation of the base metal. Spallation and/or delamination may also affect cooling hole geometry and thus effectiveness of the cooling holes.
A turbine 10 is typically operated for extended periods and is inspected at periodic intervals to check for wear, damage and other undesirable conditions that may have occurred with respect to various internal components. For example, the cooling holes are inspected to determine if any are blocked. In addition, the TBC/BC layers are inspected to determine the degree of spallation and/or delamination of the TBC/BC layers (i.e. remaining thickness of the layers) and other undesirable conditions. In order to inspect components within the turbine 10, the turbine 10 is shut down and allowed to cool down, which takes a substantial amount of time. An inspection/evaluation team must then remove hardware from the turbine 10, such as an outer casing, in order to gain access to a turbine component (for example, a stage 1 or stage 2 vane or blade). The turbine component is then removed and may be sectioned in order to be able to visually inspect the cooling holes and/or the TBC and BC layers. Ultimately, the sectioned turbine component is replaced with a new turbine component. However, the current procedure is labor intensive, time consuming and expensive.